The present invention relates to a cooling circuit, a cold drying installation, and a method for controlling a cooling circuit.
Water generally has to be removed from compressed gas, such as compressed air, before being supplied to a pneumatic network because the moisture in the gas can be harmful for the components and tools in the pneumatic network, as moisture can lead to corrosion or the accumulation of water in the tools that are not designed for this.
A known technique for drying gas is known as cold drying, and this technique is based on the principle that by cooling the gas, moisture is evacuated from the gas that is saturated or partly saturated with water, because the moisture condenses and is removed as condensed water, after which the gas is heated up again so that it is no longer saturated and thus dryer.
With cold drying a device is used that essentially consists of a closed cooling circuit that comprises a coolant that can be driven around the circuit by one or more parallel compressor(s), and which further comprises, successively in the flow direction of the coolant, a condenser that connects to the output of the compressor; an expansion valve followed by an evaporator that connects to the input of the aforementioned compressor(s), whereby the evaporator forms the primary section of a heat exchanger, and this heat exchanger also comprises a secondary section through which the gas to be dried is guided.
By entire or partial evaporation of the coolant in the evaporator, as is known, heat is extracted from the gas to be dried that flows through the secondary section, whereby this gas to be dried is cooled such that condensate is released that can be separated out, after which the gas is further dried by heating it up again.
In order to prevent damage to the compressor(s), no liquid coolant may get in because liquid coolant can damage the compression chamber and can also take the place of the oil in the compressor thereby causing accelerated wear or the bearing can seize.
For this reason, and with observance of a safety margin, traditionally it is ensured that the coolant at the outlet of the evaporator is slightly superheated with a superheating temperature of approximately 5° C. for example.
Superheating means that the temperature of the coolant in a certain place is higher than the condensation temperature, whereby the vapour pressure of the coolant is equal to the pressure in the cooling circuit in the same place. This pressure is not constant, and thus the said condensation temperature is not either.
The extent of superheating must be limited because the higher the average temperature in the primary section of the heat exchanger, the lower the heat exchanging capacity, as the temperature at the outlet of the evaporator becomes higher.
With a higher temperature of the coolant the energy-efficiency of the compressor(s) is also lower, and there is a risk that the design limits for the temperature at the outlet of the compressor(s) will be exceeded.
In order to control the extent of superheating, traditionally the expansion valve of an evaporator is controlled for a limited extent of superheating at the evaporator outlet. If the extent of superheating becomes greater than a certain target value, the expansion valve is opened such that more coolant gets into the evaporator and the superheating is reduced. If the superheating is less than the aforementioned target value, the expansion valve is controlled in the opposite direction and thus closed in other words.
Especially for cold drying installations with a high capacity, it is desirable to divide up the cooling circuit into a number of parallel sub-circuits and to operate with more than one heat exchanger.
The main reason for this is that heat exchangers can only be built for a reasonable price up to a certain heat-exchanging capacity, and also that large heat exchangers do not generally present optimum operation because a good distribution of coolant over the heat exchanger(s) is difficult to realise.
In this case there can be a number of heat exchangers, each with their own expansion valve, primary section and secondary section, placed in parallel. The various sub-flows of the gas to be dried that flow through the respective secondary sections of the heat exchangers, normally, but not necessarily, come back together again after cooling. In practice the flow rates through the various secondary circuits are approximately equal to one another.
The control of the superheating is hereby problematic, because the control of an expansion valve, in order to control the superheating at the outlet of the evaporator belonging to it, has effects on the coolant flow rates through the other expansion valves, and thus the extent of superheating in the other evaporators belonging to these expansion valves.
As a result an unstable control situation is obtained that leads to a fluctuating level of superheating and fluctuating temperatures at the outlet of the secondary sections of the heat exchangers. These temperatures, that are also called the lowest air temperature or “LAT” of a heat exchanger, can also present mutual variations. A stable uneven situation with individual LAT values that differ from the set point is also possible.
The gas, cooled in the various secondary circuits, thus has a time-varying temperature that is also not the same in the various secondary circuits.
The unstable situation has a negative impact on the temperature to be reached by the gas to be dried in the secondary sections of the heat exchangers, because too high a LAT in one secondary circuit cannot be compensated by a lower LAT in another secondary circuit. This is due to the fact that the desired LAT is typically only a few degrees above the freezing point of water, and thus an individual LAT may not normally be lower than a target value to a avoid the risk of freezing.
With a relatively small number of evaporators, for example four, and with a selection of specific coolants, the control problem is limited in practice, but indeed measurable.
With a wide choice of coolants and more than four evaporators, for example, however this problem prevents the concrete application of cool-drying installations with parallel evaporators.